1. Field of the Invention
This invention relates to an over-lock sewing machine including sewing needles attached to a lower end of a needle bar and upper and lower loopers forming stitches in cooperation with the needles.
2. Description of the Related Art
Over-lock sewing machines are generally provided with a fixed blade and a moving blade in addition to needles and upper and lower loopers. Ends of a workpiece cloth to be sewn are cut by the fixed and moving blades immediately before a sewing operation. In conventional over-lock sewing machines of the above-described type, the needles or needle bar, the upper and lower loopers and the fixed and moving blades are usually driven by a single sewing machine motor. The over-lock sewing machine forms stitches on a cloth end by the needles and the upper and lower loopers, cutting the cloth end by the fixed and moving blades, thereby carrying out an over-edge chain stitch. The over-lock sewing machine also carries out a multi-thread chain stitch or double-chain stitch in which the lower looper is reciprocally moved back and forth between two Bides of the needles with the upper looper being stopped at a predetermined stop location.
Japanese Laid-open Patent Publication No. 62-16785 (1987) discloses one of the over-lock sewing machines constructed as described above. The disclosed over-lock sewing machine comprises an operation switching mechanism for switching the upper looper between a rockable state in which the upper looper is allowed to rock and a non-rockable state in which the upper looper is stopped at a stop location. The operation switching mechanism further switches the lower looper between a first state in which a reciprocal movement of the lower looper is disallowed and a second state in which the reciprocal movement of the lower looper is allowed. The operation switching mechanism includes a mechanism for coupling or decoupling a part of a transmission mechanism transmitting a driving force from a sewing machine motor to the upper and lower loopers, thereby switching, when an operation lever is operated, between the state in which the driving force from the motor is transmitted to the upper and lower loopers and the state in which the driving force is not transmitted to the upper looper.
Furthermore, Japanese Laid-open Patent Publication No. 7-659 (1995) discloses an over-lock sewing machine provided with an operation switching mechanism for switching between a state in which the upper looper is rockable and a state in which the upper looper is stopped at a stop location to thereby be non-rockable. The operation switching mechanism further switches the moving blade between an operative position where the driving force of the motor is transmitted to the moving blade so that the moving blade is moved upward and downward to thereby cut the cloth end in cooperation with the fixed blade, and an inoperative position where the driving force of the motor is not transmitted to the moving blade. This operation switching mechanism is provided with a mechanism for coupling or decoupling a part of a transmission mechanism transmitting a driving force from the motor to the upper looper and the moving blade when a switching lever is operated, thereby switching between the state in which the driving force from the motor is transmitted to the upper looper and the moving blade and the state in which the driving force is not transmitted to the moving blade.
In the over-lock sewing machines disclosed in the above-mentioned Japanese publications, the needle bar, the upper and lower loopers, and the moving blade are driven by a single motor. Accordingly, this driving manner requires the operation switching mechanism. However, the operation switching mechanism necessitates a large number of parts and accordingly has a complicated structure. The operating switching mechanism increases the sizes of the conventional over-lock sewing machines, resulting in an increase in the manufacturing cost of the sewing machines. Furthermore, adjusting an amount of operation and operation timings of the upper and lower loopers etc. driven by the single motor is difficult. Moreover, in the over-lock sewing machine disclosed in publication No. 7-659, the moving blade is also switched from the operative position to the inoperative position when the upper looper is switched from the rockable state to the non-rockable state. Accordingly, execution of the over-edge chain stitch is always accompanied by the cutting of the cloth end, whereas the double-chain stitch is always executed without cutting the cloth end. Consequently, the over-edge chain stitch cannot be executed without cutting the cloth end, and the double-chain stitch cannot be executed with the cloth end being cut.